1. Field of the Invention
The present invention relates to an organic light emitting display, and more particularly, the present invention relates to an organic light emitting display suitable for a high quality and high resolution display device, by rapidly charging a data voltage using a voltage programming technique, after compensating for deviations, such as a threshold voltage and a mobility of a driving transistor, using a current programming technique.
2. Description of the Related Art
Generally, a conventional organic light emitting display electrically excites a phosphor or a phosphorescent organic compound and emits light, which displays an image by driving N×M organic emitting cells. The organic light emitting cell of FIG. 1 includes an anode of Indium Tin Oxide (ITO), an organic thin film and a cathode (metal). The organic thin film is a multilayer structure including an EMitting Layer (EML), an Electron Transport Layer (ETL) and a Hole Transport Layer (HTL), and may further include an extra Electron Injection Layer (EIL) and Hole Injection Layer (HIL).
Techniques for driving the organic light emitting cell include a simple matrix technique and an active matrix technique using a Thin Film Transistor (TFT) or a MOSFET. The simple matrix technique drives a light emitting cell by forming an anode to intersect with a cathode and selecting a line. The active matrix technique connects the TFT and a capacitor to respective Indium Tin Oxide (ITO) pixel electrodes to maintain a voltage by the capacity of a capacitor. The active matrix technique is divided into a voltage programming technique and a current programming technique according to the form of a signal supplied to the capacitor for maintaining the voltage.
Organic light emitting displays using voltage and current programming techniques are respectively explained below with reference to FIGS. 2 and 3.
FIG. 2 is a pixel circuit of a voltage programming technique for driving an Organic Light Emitting Diode (OLED) and representatively illustrates one of N×M pixel circuits.
Referring to FIG. 2, a driving transistor (M1) is coupled to the OLED so as to supply a light-emitting current. The amount of current of the driving transistor (M1) is controlled by a data voltage supplied through a first switching element (S1). A first capacitive element (C1) for maintaining the supplied voltage for a fixed period of time is coupled between a gate and a source of the driving transistor (M1). A first electrode of the first switching element (S1) is coupled to a data line (Data[m]), and a control electrode thereof is coupled to a scan line (Scan [n]).
When the first switching element (S1) is turned on by a scan signal supplied to the control electrode of the first switching element (S1), a data voltage is supplied from the data line (Data[m]) to the control electrode of the driving transistor (M1). As a result thereof, a current (IOLED) corresponding to a voltage (VGS) charged between the gate and the source of the driving transistor (M1) by the first capacitive element (C1) flows to the drain of the driving transistor (M1) and the OLED emits light according to the current (IOLED).
The current flowing to the OLED is obtained by Equation 1.
                                                                        I                OLED                            =                                                β                  2                                ⁢                                                      (                                                                  V                        GS                                            -                                              V                        TH                                                              )                                    2                                                                                                        =                                                β                  2                                ⁢                                                      (                                                                  V                        SG                                            -                                                                                                V                          TH                                                                                                              )                                    2                                                                                                        =                                                β                  2                                ⁢                                                      (                                                                  V                        DD                                            -                                              V                        DATA                                            -                                                                                                V                          TH                                                                                                              )                                    2                                                                                        Equation        ⁢                                  ⁢        1            
In Equation I, IOLED is a current flowing to the OLED, VGS is a voltage between the gate and the source of the driving transistor (M1), and a VTH is a threshold voltage of the driving transistor (M1), a VDATA is a data voltage, and β is a constant.
As shown in Equation 1, according to the pixel circuit shown in FIG. 2, the current corresponding to the supplied data voltage is supplied to the OLED, and the OLED emits light corresponding to the supplied current.
In the pixel circuit of the voltage programming technique discussed above, the luminance is non-uniform due to deviations in mobility and threshold voltage of a TFT caused by non-uniformities in the manufacturing process.
On the contrary, the pixel circuit of the current programming technique may obtain a uniform display characteristic, even though the driving transistor in the respective pixels has a non-uniform voltage-current characteristic, if a current source supplying a current to the pixel circuit is uniform on all of the data lines.
FIG. 3 is a pixel circuit of a current programming technique for driving the OLED, and representatively illustrates one of N×M pixel circuits.
Referring to FIG. 3, the driving transistor (M1) is coupled to the OLED so as to supply a light-emitting current, and the amount of current of the driving transistor (M1) is controlled by a data current supplied through the first switching element (S1).
When the first and second switching elements (S1 and S2) are turned on due to a selection signal outputted from the scan line (Scan [n]), the driving transistor (M1) is connected in a diode configuration, a voltage corresponding to a data current (IDATA) from the data line (Data [m]) is stored in the first capacitive element (C1), the current (IOLED) corresponding to the voltage stored in the first capacitive element (C1) flows to the drain of the driving transistor (M1), and the OLED emits light corresponding to the current (IOLED). The current flowing to the OLED is obtained by Equation 2.
                              I          OLED                =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                      V                    TH                                                  )                            2                                =                      I            DATA                                              Equation        ⁢                                  ⁢        2            
In Equation 2, IOLED is a current flowing to the OLED, VGS is a voltage between the gate and the source of the driving transistor (M1), VTH is a threshold voltage of the driving transistor (M1), IDATA is a data current, and β is a constant.
As shown in Equation 2, in accordance with the current programming pixel circuit discussed above, the current (IOLED) flowing to the OLED is the same as the data current (IDATA), so that a programming current source may obtain a uniform characteristic on all of the panels. However, the current (IOLED) flowing to the OLED is a minute current, and the pixel circuit is controlled by the minute current (IDATA), so that it has a problem in that it takes a considerable amount of time to charge the data line. For example, if a load capacitance of the data line is 30 pF, several milliseconds are needed to charge a load of the data line with a data current of several tens to several hundreds of nA. Therefore, there is a problem in that there is not sufficient time to charge the load of the data line, considering a line time of several tens of μs.